Although the conventional cathode for a cathode ray tube has employed a base cathode structure of nickel (Ni) containing a trace of reducing material coated with an electron emissive substance, recently a cathode of an impregnate type is widely used in order to accommodate to the recent trend that an electron tube has a high resolution and a large screen size requiring a high intensity of the cathode currents. The impregnate type cathode may carry a high cathode current, but should be made of a refractory metal such as molybdenum (Mo) due to the high operating temperature of about 1050.degree.-1200.degree. C., and moreover suffers an adverse volume change because of the high temperature.
The cathode structure of an impregnate type generally comprises, as shown in FIG. 1, a pellet 11 of refractory porous sintered body impregnated with an electron emissive substance, a cathode cup 12 of a refractory metal for containing the pellet, a cylindrical cathode sleeve 13 of a refractory metal for receiving the cathode cup 12 in the upper opening, a cathode holder 14 for holding the cathode sleeve 13 outwardly, a plurality of metal ribbons 15, 15' and 15" for fixing the cathode sleeve 13 to the cathode holder 14, the cathode support 16, and a heater 17 mounted inside the cathode sleeve.
Ends of the metal ribbons are fixed to the lower end of the cathode sleeve 13, and the other ends to the upper end of the cathode holder 14, so that the cathode sleeve 13 is held by the cathode holder 14.
The cathode holder 14 is supported in the cathode supporter 16 of an electron gun assembly.
In another embodiment of the conventional cathode structure, the pellet 11 of refractory porous sintered body is set in a cathode ring attached on the upper closed end of a cathode sleeve.
In this conventional cathode structure, the reason that the metal ribbons 15 connect the cathode sleeve 13 and cathode holder 14 is to prevent the heat transferred from the heater 17 to the cathode sleeve 13 from being further transferred to the cathode holder 14. Namely, it is very important to prevent the heat generated by the heater 17 from being lost towards the outside because the time being taken for the image to appear on the screen of a cathode ray tube depends upon the electron emission of the impregnate pellet 11 that is caused by the heat transferred from the heater 17 to the impregnate pellet 11. Furthermore, if the heat lose is reduced, the power consumption of the heater may also be reduced together with the thermal distortion of the cathode structure, thus achieving a high luminance and resolution of the screen image.
To achieve this end, there has been proposed the blackening of the cathode sleeve 13 for increasing the heat radiation. The blackening of the cathode sleeve 13 is generally achieved by oxidizing the chromium (Cr) component of the cathode sleeve made of Ni-Cr alloy in a wet hydrogen atmosphere at a temperature of about 1100.degree. C. However, the conventional impregnate cathode structure wherein the cathode sleeve 13 is fixed to the cathode holder 14 by means of the metal ribbons 15 induces problems related to the thermal distortion in a cathode sleeve 13 of Ni-Cr alloy. Namely, since the melting point of the Ni-Cr alloy is about 1400.degree. C. and the activation temperature of the electron emissive substance is about 1200.degree. C., the high operating temperature of the electron emissive substance easily causes the thermal distortion of the cathode sleeve 13 of Ni-Cr alloy.
Moreover, since the lower end of the cathode sleeve is fixed to the cathode holder, the thermal expansion of the cathode sleeve caused by the high operating temperature of the electron emissive substance occurs upwardly in the direction of the impregnate pellet 11 changing the gap between the impregnate pellet 11 and the first electrode of an electron gun (not shown) opposing it, thereby the cathode current is changed to cause the variations of the screen image of a cathode ray tube.
Hence, it is not suitable to employ the cathode sleeve of Ni-Cr alloy in the conventional impregnate cathode structure. There has been proposed a further embodiment of the conventional cathode structure wherein the cathode sleeve is made of a refractory metal such as molybdenum (Mo) and tantalum (Ta) in order to satisfy the requirements of the high cathode current. However, the cathode sleeve made of such refractory metal is very difficult to blacken for reducing the time being taken for the image to appear on the screen, and needs a relatively high cost.